corporate: Add end_date to fixed-price plans.

Earlier, we were not configuring the end_date while creating
a fixed-price plan which would result in the automatic renewal
of the plan at the end of billing cycle.

Our plan is to re-evaluate the pricing when we are close to the
end date, then:
1. Schedule a new fixed-price plan with the updated or same price.
2. or Don't do anything - the plan will end on the end_date.

Now, we add an end_date of 1 year from the start date when
creating a fixed-price plan.

corporate/migrations/0039_backfill_end_date_for_fixed_price_plans.py

@@ -0,0 +1,63 @@
+# Generated by Django 4.2.9 on 2024-02-21 11:53
+
+from datetime import datetime
+
+from django.db import migrations
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def add_months(dt: datetime, months: int) -> datetime:
+    assert months >= 0
+    # It's fine that the max day in Feb is 28 for leap years.
+    MAX_DAY_FOR_MONTH = {
+        1: 31,
+        2: 28,
+        3: 31,
+        4: 30,
+        5: 31,
+        6: 30,
+        7: 31,
+        8: 31,
+        9: 30,
+        10: 31,
+        11: 30,
+        12: 31,
+    }
+    year = dt.year
+    month = dt.month + months
+    while month > 12:
+        year += 1
+        month -= 12
+    day = min(dt.day, MAX_DAY_FOR_MONTH[month])
+    # datetimes don't support leap seconds, so don't need to worry about those
+    return dt.replace(year=year, month=month, day=day)
+
+
+def backfill_end_date_for_fixed_price_plans(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    CustomerPlan = apps.get_model("corporate", "CustomerPlan")
+    CustomerPlan.ACTIVE = 1
+
+    fixed_price_plans = CustomerPlan.objects.filter(
+        status=CustomerPlan.ACTIVE, fixed_price__isnull=False
+    )
+    for plan in fixed_price_plans:
+        assert plan.end_date is None
+        assert plan.billing_cycle_anchor is not None
+        plan.end_date = add_months(plan.billing_cycle_anchor, 12)
+        plan.save(update_fields=["end_date"])
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("corporate", "0038_customerplanoffer_sent_invoice_id_invoice"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            backfill_end_date_for_fixed_price_plans,
+        ),
+    ]